<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<body>
   <button>发送</button> 
   <script>
        const btn = document.getElementsByTagName('button')[0];
        let x = null;
        let isSending = false;

        btn.onclick = function() {
            if(isSending) x.abort();//如果正在发送，则取消该请求 
            x = new XMLHttpRequest();
            isSending = true;
            x.open('GET', 'http://127.0.0.1:8000/delay');
            x.send();
            x.onreadystatechange = function() {
                if(x.readyState ===4) {
                    isSending = false;
                }
            }
       }
   </script>
</body>
</html>